Find the odd geometric figure (polygons vs non-polygon): Choose the figure that is not a polygon with straight sides.

Introduction / Context:
Geometric classification problems typically hinge on essential defining properties. Polygons are 2D shapes composed of straight line segments joined end to end. A circle, in contrast, has no straight sides or vertices; its boundary is a continuous curve equidistant from a center. This crisp, definitional difference reliably identifies the outlier.

Given Data / Assumptions:

• Triangle: polygon with 3 straight sides and 3 vertices.
• Rectangle: quadrilateral polygon with opposite sides equal and all angles 90 degrees.
• Square: quadrilateral polygon with all sides equal and all angles 90 degrees.
• Circle: closed curve with infinitely many points, all at a constant radius from a center; no vertices, no straight edges.

Concept / Approach:
Test each option against the polygon definition: straight sides and vertices are mandatory. Any figure lacking these is not a polygon. Shapes like triangle, rectangle, and square satisfy the polygon criteria. A circle does not.

Step-by-Step Solution:

Verification / Alternative check:
Consider interior angles: polygons have a finite set of interior angles at vertices; a circle has no vertices, hence no interior angles in this sense.

Why Other Options Are Wrong:
Triangle, rectangle, and square all meet the strict polygon definition based on straight sides and vertex angles.

Common Pitfalls:
Do not rely on symmetry or number of sides to split the set; the curved-vs-straight boundary distinction is the simplest and most decisive test.

Final Answer:
Circle